1. Field of the Invention
The present invention relates to an apparatus for continuously producing a substrate sheet for optical recording media used in optically recording and reproducing information using a laser beam or the like. It also relates to a process for producing a substrate sheet for optical recording media making use of the apparatus.
2. Related Background Art
In optical recording media such as optical disks and optical cards, optically detectable minute pits of several .mu.m in diameter are commonly formed in the form of a track on a thin recording layer provided on a substrate, and thus information can be recorded at a high density. In such optical recording media, a laser beam must be scanned along the track when the information is recorded and reproduced. For this purpose, a substrate with guide grooves for tracking is commonly used.
Methods of preparing such guide grooves or a substrate provided with guide grooves are known to include a compression molding process in which a softened plastic material is pressed using a stamper followed by curing, an injection molding process in which a molten plastic material is injected into a mold provided with a stamper followed by curing, and a 2P (photo-polymerization) process in which guide grooves are transferred from a stamper by the use of a photopolymer. Of these, the 2P process is one of the superior processes for preparing substrates in view of the advantages that guide grooves can be readily transferred, substrates have a good solvent resistance, and substrates can be made to have less optical distortion.
More specifically, the superior solvent resistance of a substrate is advantageous when a recording layer is formed by coating, using an organic material as a recording material. For example, when a substrate is prepared by the 2P process, it is possible to use a solvent that can not be used in a substrate comprised of the usual plastics such as acrylate and polycarbonate because of its power to dissolve the surface of the substrate. Hence, it becomes possible to use a recording material having a good solubility with respect to the solvent, so that the recording material can be selected from a vast range of materials. The 2P process also has a great advantage in that it requires only a very small investment in equipment, compared with other processes
Compared with other processes, however, the 2P process requires a longer production time per sheet of substrates, and is not suited for mass-production. Under such circumstances, a proposal, as disclosed in PCT Translation Publication No. 62-506504, has been made about a technique by which, using a master on which patterns of pits or grooves have been formed, the patterns are transferred and formed on a film substrate so that substrates for film-like optical recording media can be continuously produced.
A conventional method for producing substrates for optical recording media will be described below with reference to the accompanying drawings. FIG. 8A illustrates the structure of an apparatus used in forming guide grooves on a substrate according to a conventional method. FIG. 8B is a partial illustration of a roll stamper shown in FIG. 8A, viewed from the side on which an ultraviolet lamp 6 is provided. In these drawings, a substrate sheet 1 fed from a feed roll 11 is delivered along the circumference of a roll stamper 3 on which guide grooves or preformat patterns corresponding with information are formed. A nip roll 12 has the function of pressing the substrate sheet 1 against the surface of the roll stamper 3. A resin tank 13 containing a liquid ultraviolet-curable resin, curable as a result of exposure to ultraviolet rays, is provided beneath the roll stamper 3. In this resin tank 13, a coating roll 4 is provided which rotates in pressure contact with the roll stamper 3. The liquid ultraviolet-curable resin 2 is coated by this coating roll 4 on the surface of the roll stamper. The ultraviolet-curable resin 2 thus coated is held between the substrate sheet 1 and the roll stamper 3 by the action of the nip roll 12. An ultraviolet lamp 6 is provided above the nip roll 12, and the ultraviolet-curable resin 2 held between the substrate sheet 1 and the roll stamper 3 is irradiated with ultraviolet rays so that the ultraviolet-curable resin is cured. A delivery roll 14 has the function of peeling from the roll stamper 3 the substrate sheet 1 and the layer of the ultraviolet-curable resin having been cured in a fixed state to the substrate sheet. Thus, preformat patterns of optical disks are transferred to the substrate sheet.
Here, as commonly inherent in ultraviolet-curable resins, the ultraviolet-curable resin exhibits flow behavior before it cures, and ultraviolet-curable resins usually used cure as the reaction proceeds according to radical polymerization, where they are inhibited from curing because of the presence of oxygen in the air.
For this reason, in the above method of producing substrates, a side edge 15, as shown in FIG. 8B, of the ultraviolet-curable resin layer held between the substrate sheet and the roll stamper is inhibited from curing because of the oxygen in the air even after irradiation with ultraviolet rays, resulting in insufficient curing of the ultraviolet-curable resin at that part. Hence, the ultraviolet-curable resin at the side edge exhibits flow behavior, and it often occurs that uncured ultraviolet-curable resin protrudes from an edge of the substrate sheet 1 or the resin at the side edge is not fixed to the substrate sheet 1 even if the coating roll 4 for the ultraviolet-curable resin is made to have a smaller width than the width of the substrate sheet 1. Such uncured or semi-cured ultraviolet-curable resin remains on and adheres to the surface of the roll stamper after the substrate sheet is peeled from the roll stamper. The ultraviolet-curable resin thus adhering to the stamper is cured and built up as a result of irradiation with light while the molding operation is repeated. The ultraviolet-curable resin thus cured and built up is caught on the coating roll 4 or a blade 5 and comes off as scraps, which adhere to the substrate sheet and are included into it. This has been one of the causes of recording and reproducing errors on recording media.